Trypanosoma cruzi is a protozoan that causes Chagas' disease in humans. It divides in th midgut of insect vectors until it transforms into infective metacyclic forms. In the mammalian host the parasite divides intracellularly and, when the cells lyse, trypomastigote forms are released into the blood stream. Trypomastigotes emerging from the host cells contain little or no sialic acid, but carry on their surface a trans-sialidase enzyme (TS). TS rapidly transfers sialic acid from host glycoconjugates to the surface of blood-derived trypomastigotes, forming a stage-specific epitope (Ssp3). Metacyclics also acquire sialic acid from the mammalian host, but their sialylated molecules and TS are structurally different. T. cruzi does not synthesize sialic acid, and the high efficiency of sialic acid acquisition suggests that this is an important process in completing its life cycle. Some evidence suggests that sialic acid is important for cell invasion. It could be involved in escape from athe host's immune response, and in directing the parasite to defined tissues. In this proposal we aim to understand the role of TS and the sialic acid acceptors. In the parent grant the main focus was on the TS and sialic acid acceptors of mammalian forms of the parasite. Here we will concentrate on the insect-derived forms of T. cruzi, which are the forms that first enter into contact with the human host in natural infections. We will: 1. Isolate and characterize TS from metacyclic forms. We will determine whether in those forms TS is developmentally regulated, and if it has any resemblance to sialidase or TS from trypomastigotes We plan to identify, purify and clone CDNA that codes for TS in the insect- derived forms of T. cruzi. 2. Identify the acceptor(s) for sialic acid in the insect-derived forms of T. cruzi and determine whether the sialic acid-containing molecules are involved in invasion and resistance to the host immune attacks.